In the fields using electromagnetic motors such as robots, cutting machines, automobiles, etc., demand has been mounting to reduce the weight of driving systems. However, because the power densities of the electromagnetic motors depend on the weight of motors, the weight reduction of actuators utilizing the electromagnetic motors is limited. To obtain large power with reduced size and weight, actuators without using electromagnetic motors are desired.
As actuators, which can be made smaller in size and weight, polymer actuators have recently been attracting much attention. Known as the polymer actuators are a gel actuator using a conductive polymer gel, a polymer membrane actuator using a conductive polymer membrane, etc.
An example of the conductive polymer membrane actuator may be a conductive polymer membrane and metal electrodes attached to their surfaces. The metal electrodes are formed on the conductive polymer membrane by such methods as chemical plating, electroplating, vapor deposition, sputtering, coating, pressure-bonding, welding, etc. When potential difference is provided to an assembly of a conductive polymer membrane and metal electrodes in a water-containing state, the conductive polymer membrane bends and deforms, and the assembly can be utilized as a driving force.
A phenomenon generating stress such as strain, etc. by applying voltage, like an actuator, is called “negative piezoelectric effect,” and a phenomenon generating voltage by applying stress such as pressure, tension, etc., is called “positive piezoelectric effect.” It is known that the piezoelectric body showing a negative piezoelectric effect also shows a positive piezoelectric effect. It is considered that a combination of negative and positive piezoelectric actuators is efficiently driven. For example, when voltage is applied to one actuator, it turns the electric energy to movement, which is transmitted to the other actuator. Part of the transmitted kinetic energy can be recovered as electric energy by the other actuator. Such an actuator utilizing both positive and negative piezoelectric effects is very efficient.
JP8-36917A describes a piezoelectric element produced by stretching a ferroelectric copolymer film of trifluoroethylene and vinylidene fluoride, and pauling the resultant ferroelectric, single-crystal film with a paraelectric or hexagonal phase while keeping the surface of the stretched film substantially free. When voltage is applied to this piezoelectric film with metal electrodes bonded to both sides in a water-containing state, the piezoelectric film extends or contracts on one side, so that it curves as a polymer film actuator. However, it is difficult to control the displacement amount and position of a bending actuator. In the case of the positive piezoelectric effect, it is difficult to control electric energy by the displacement amount and position. In addition, this actuator is not efficient because of failing to obtain high electric energy corresponding to the stress applied.
JP2001-294642A describes a polyurethane elastomer-based actuator produced from polyisocyanate and at least one selected from the group consisting of polyols having amide groups and polyols comprising amide-group-containing polymers. When an electric field is applied to the polyurethane elastomer-based actuator in a state where it contains 0.01-50% by weight of moisture, the polyurethane elastomer exhibits amplified strain in an electric field direction. It also describes that when voltage is applied to a 2-mm-thick polyurethane elastomer sheet sandwiched by electrodes and containing water, the sheet undergoes thickness change by strain. However, this polyurethane elastomer-based actuator is inefficient because it utilizes small thickness change as displacement. On the other hand, when the positive piezoelectric effect is to be used, it is difficult to achieve efficiently controllable energy conversion, because very small displacement should be used to generate electric energy, resulting in the likelihood that too large a force is applied to the sheet.
As discussed above, although piezoelectric polymers constituting actuators generate electric energy by stress change, the conventional polymer actuators mostly in the form of films are not suited to generate electric power by stress change. Polymer actuators capable of using the positive piezoelectric effect effectively have not been known yet.